This invention relates generally to air conditioning systems and, more particularly, to a method and apparatus for varying the speed of a compressor in an air conditioning system to control the discharge temperature thereof.
Air conditioning and heat pump systems of the type commonly used for residential and light commercial application are commonly driven by hermetic compressors. In such a system it is desirable to provide a means for protecting the compressor from overload conditions which may occur, for example, in extreme weather conditions, by improper use of the system, or by a malfunction of certain components in the system. Typically this protection has been provided by a sensing of the discharge pressure of the compressor and a shutting down of the system in the event of extreme pressures being detected.
Variable speed motors are now available and are being used for driving not only the compressor but also the fans for both the outdoor and indoor coils. With this flexibility in operating speeds, it is possible to obtain higher operating efficiencies over variable load and ambient conditions. Further, in a case of the compressor drive motor, rather than shutting the compressor down when excessive discharge pressures are sensed, it is possible to simply reduce the compressor speed in order to relieve the compressor from the excessive operating condition.
If the suction pressure to the compressor remains constant, the discharge pressure will be representative of the discharge gas temperature. However, if the suction pressure varies, as may occur because of changes in the ambient or system conditions, then the operating temperature of the compressor may become excessive, even though the discharge pressure has not exceeded the allowed threshold limit. For example, if there is a loss of refrigerant by way of leakage or the like, the suction pressure will go down and the discharge temperature will tend to go up. Similarly, if there is a restriction in refrigerant flow as may be caused by a kink in the refrigerant line, the suction pressure will be reduced and the discharge temperature will tend to rise. Further, as the temperature of the ambient air flowing across the evaporator is reduced, suction pressure is reduced and the discharge temperature will tend to rise.
If the operating temperature of the compressor becomes excessive, damage maybe caused in various ways. For example, when exposed to relatively high temperatures, the insulation in the motor may degrade and eventually cause a short circuit. Further, at high temperatures the lubricating oil tends to break down and that, in turn, will cause excessive wear and possible failure of various components within the compressor.
It is therefore an object of the present invention to provide an improved method and apparatus for protecting a compressor from overtemperature conditions.
Another object of the present invention is the provision for protecting a variable speed compressor from overtemperature conditions.
Yet another object of the present invention is that of protecting a variable speed compressor from overtemperature conditions during periods when the discharge pressure does not become excessive.
Yet another object of the present invention is the provision for a variable speed compressor control system which is economical to manufacture, and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.